The present invention relates to an organic electroluminescent image display apparatus, especially to an organic electroluminescent image display apparatus in which light can be taken out on a cathode layer side of an upper surface.
An organic electroluminescent (EL) device has advantages that visibility is high by self color development, an all-solid display superior in impact resistance is provided different from a liquid crystal display, a speed of response is high, little influence of a temperature change is exerted, and a visual field angle is large. In recent years, use as a light emitting device in an image display apparatus has been noticed.
For a constitution of the image display apparatus using the organic EL device, a constitution is usually employed in which a stacked structure of an anode layer/light emitting layer/cathode layer is basic and a transparent anode layer is formed on a substrate using a glass substrate. In this case, an emitted light is taken out on a substrate side (anode layer side).
In recent years, attempts (upper surface emission) have been made to form a transparent cathode layer and to take out the emitted light on the cathode layer side. When this upper surface emission is realized, and when not only the cathode layer but also the anode layer are formed to be transparent, it is possible to constitute a light emitting device that is entirely transparent. An arbitrary color can be used as a background color of this transparent light emitting device, and display can be colorful at a time other than an emission time, and a decorative property is improved. On the other hand, when black is used as the background color, contrast at the time of the emission is enhanced. When the upper surface emission is realized, and when a color filter or a color conversion layer is used, each layer can be disposed on the light emitting layer. Furthermore, since the emission is not blocked off by a thin film transistor (TFT) of an active driving display apparatus, the display apparatus having a high numerical aperture is possible.
As an example of the organic EL image display apparatus in which the cathode layer is formed to be transparent and accordingly the above-described upper surface emission is possible, a constitution has been disclosed in which an organic layer including an organic EL light emitting layer is disposed between the anode layer and the cathode layer, the cathode layer is constituted of an electron injection metal layer and an amorphous transparent conductive layer, and the electron injection metal layer contacts the organic layer (Japanese Patent Application Laid-Open No. 10-162959). Moreover, in order to prevent a cathode material from being diffused in the organic layer, a constitution has been disclosed in which a Ca diffusion barrier layer is disposed between the cathode layer and the organic layer, and short circuit of the organic EL device and degradation of characteristics are prevented (Japanese Patent Application Laid-Open No. 10-144957). As an example of opposite surface emission, a constitution has been disclosed in which conductive layers such as Ag, Mg, and TiN are disposed between the transparent cathode layer and the light emitting layer in order to reduce resistance of the transparent cathode layer (Japanese Patent Application Laid-Open No. 10-125469). Furthermore, a constitution has been disclosed in which TiN is used in the anode layer for a purpose of preventing penetration or diffusion of oxygen or indium into the organic layer (Japanese Patent Application Laid-Open Nos. 2002-15859, 2002-15860). In order to enhance a light emitting efficiency, the electron injection layer doped with a metal is disposed between the organic layer and the cathode layer to stabilize injection of electric charges into the organic layer (Japanese Patent Application Laid-Open Nos. 10-270171, 10-270172).
However, in the conventional organic EL image display apparatus in which the upper surface emission is possible, oxidation of the organic layer or the electron injection layer cannot be avoided by introduction of oxygen in a step of forming the transparent cathode. Therefore, there have been problems that the characteristics of the organic layer or the electron injection layer are degraded, dark spots are generated, and high-quality image display cannot be obtained.
Moreover, in the conventional organic EL image display apparatus in which the upper surface emission is possible, there have been problems that an impact by an argon ion having a high irradiance is added to the organic layer at several hundreds of volts in a step of forming the transparent cathode layer in a sputtering method, the structure of the organic layer changes, and non-radiation extinction occurs in an interface between the organic layer and the electron injection layer. There have also been problems that it is impossible to avoid the oxidation of the metal doped in the electron injection layer by the introduction of oxygen at the time of the forming of the transparent cathode layer, and therefore an electron injection function of the electron injection layer is lost.